1. Field of the Invention
The present invention relates to a single crystal pulling apparatus for pulling a single crystal of a semiconductor from a semiconductor melt stored in a double crucible.
2. Description of the Related Art
The CZ growth technique is an example of one of the currently known methods for growing single crystals of semiconductors such as silicon (Si) or gallium arsenide (GaAs).
Because this CZ growth technique enables simple generation of large diameter, high purity single crystals which are free from dislocation or have extremely low levels of lattice defect, it is widely used in the growing of a variety of semiconductor crystals.
In recent years, the demand for larger diameter, higher purity single crystals with uniform levels of oxygen concentration and impurity concentration, has seen this CZ growth technique improved in various ways, to meet these demands.
One of the improvements of the aforementioned CZ growth technique which has been proposed, is a continuous magnetic field application CZ technique (hereafter abbreviated as CMCZ technique), which employs a double crucible. Features of this method are that it enables the growth of single crystals with good slip-free ratios and with extremely good control of oxygen concentration levels, by external application of a magnetic field to the semiconductor melt inside the crucible, which suppresses convection in the semiconductor melt, and that it enables the simple generation of long single crystals of the semiconducting material by allowing continuous supply of the source materials, to a position located between outer and inner crucibles. Consequently, this method is recognized as one of the best for obtaining large diameter, long single crystals of semiconducting materials.
FIG. 12 shows an example of a single crystal silicon pulling apparatus proposed in Japanese Patent Application, First Publication, No. Hei-4-305091, which employs the CMCZ technique outlined above. In this single crystal pulling apparatus 1, a double crucible 3, a heater 4, and a source material supply tube 5 are positioned inside a hollow air-tight chamber 2, and a magnet 6 is positioned outside this chamber 2.
The double crucible 3 comprises an approximately hemispherical outer crucible 11 made from quartz (SiO.sub.2), and an inner crucible 12 made from quartz, which is a cylindrical partition body which is fitted inside the outer crucible 11. The side wall of this inner crucible 12 contains a plurality of communicating apertures 13, which connect the area between the inner and outer crucibles, 12 and 11 respectively (the source material melt region), with the inside of the inner crucible 12 (the crystal growing region).
This double crucible 3 is mounted on a susceptor 15, which sits on a vertical shaft 14 located centrally at the lower portion of the chamber 2, and can be rotated in a horizontal plane at a specified angular velocity about the axis of the shaft 14. The semiconductor melt (the source material for the generation of single crystals of semiconductor, melted by heating) 21 is stored inside this double crucible 3.
The heater 4 heats and melts the semiconductor source material inside the crucible, and also maintains the temperature of the thus produced semiconductor melt 21. Normally resistance heating is used. The source material supply tube 5 is used to continuously inject a specified volume of semiconductor source material 22, on to the surface of the semiconductor melt between the outer crucible 11 and the inner crucible 12.
The magnet 6 is used to apply, externally, a magnetic field to the semiconductor melt 21 inside the double crucible 3, and to produce Lorentz forces in the semiconductor melt 21, thereby effecting control of convection within the semiconductor melt 21, the control of oxygen concentration, and the suppression of surface vibration, and so on.
Examples of the source materials 22 which can be supplied through the source material supply tube 5 mentioned above include polysilicon which has been converted to flake form by crushing in a crusher, or polysilicon granules deposited from gaseous source material using thermal decomposition, with further supply, as necessary, of elemental additives known as dopants, such as boron (B) (in the case of production of p-type single crystals of silicon) and phosphorus (P) (in the case of production of n-type single crystals of silicon).
In the case of gallium arsenide (GaAs), the operation is the same as that outlined above, but in this case, the elemental additive used is either zinc (Zn) or silicon (Si).
With the single crystal pulling apparatus 1 outlined above, a seed crystal 25 is suspended from a pulling shaft 24 located above the inner crucible 12 and over the shaft axis line, and a single crystal of semiconductor 26 is grown at the upper surface of the semiconductor melt 21 around the nucleus of the seed crystal 25.
However, as has been disclosed in Japanese Patent Application, First Publication, No. Sho-63-303894, in this type of single crystal pulling apparatus, the growing of single crystals requires that first the polycrystalline source material, such as lumps of polysilicon, be melted, and the resulting semiconductor melt 21 stored inside the outer crucible 11, with the double crucible 3 then being formed by positioning the inner crucible 12 above the outer crucible 11 and then mounting it down onto the outer crucible 11.
The reason that the double crucible 3 is formed after melting of the polycrystalline source material, is that in order to effect complete melting of the polycrystalline source material to obtain the semiconductor melt 21, the temperature of the source material inside the outer crucible 11 needs to be raised, using the heater 4, to a temperature hotter than the single crystal growing temperature. On the other hand, if the inner crucible 12 were to be mounted on the outer crucible prior to the melting stage, large thermal deformation of the inner crucible 12 would occur.
Consequently, by mounting the inner crucible 12 on the outer crucible 11 after complete melting of the source material and a subsequent lowering in the heat being applied by the heater 4, the high temperatures required in the initial source material melting stage can be avoided, and deformation of the inner crucible 12 suppressed.
Furthermore, the communicating apertures 13 of the inner crucible 12 are set at a predetermined aperture diameter small enough to ensure that when source material is added, the semiconductor melt 21 will flow only from the outer crucible 11 to the inner crucible 12. The reason for this restriction is that if the aperture area of the apertures 13 is too large so that the phenomenon arises where, through convection, the semiconductor melt is able to flow from the crystal growing region back to the source material melt region, then the control of impurity concentrations during the single crystal growth, and the control of the melt temperature would become problematic.
However, in those situations where the diameter of the communicating apertures 13 is too small, it becomes difficult for the semiconductor melt to flow freely from outside the inner crucible 12 to the inside thereof, and thus more likely for a difference to develop between the level of the semiconductor melt outside the inner crucible 12 and the level inside the inner crucible 12, which will then produce vibrations on the melt surface as the system attempts to rectify this difference in levels, which in turn exercises a deleterious influence on the single crystal growth.
As a result, the diameters of the communicating apertures 13 described above, need to be set in a range between being sufficiently small to prevent reverse flow of the semiconductor melt 21 from inside the inner crucible 12 to the outside thereof, and sufficiently large to prevent the development of a difference in levels between the semiconductor melt inside the inner crucible 12 and the semiconductor melt outside thereof.
However, in single crystal pulling apparatus like those described above, the following type of problems remain.
After storage of the semiconductor melt 21 in the outer crucible 11, the inner crucible 12 is mounted on to the outer crucible to form the double crucible, and there are occasions when gas bubbles A of a gas such as argon, which is being used as the inert gas atmosphere, can adhere to the multiple communicating apertures 13 of the inner crucible 12, as shown in FIG. 13B. In those communicating apertures 13 where gas bubbles A have adhered in this way, the effective diameter of the aperture is narrowed, which increases the melt flow resistance, hindering the flow of the semiconductor melt 21 from the outer crucible 11, through the communicating apertures 13, to the inner crucible 12. That is, the diameter of the communicating apertures 13 becomes narrower than the suitable diameter range described above, and single crystal growth becomes problematic.
Furthermore, if the diameters of the communicating apertures are increased, it could be expected that effects from adhered gas bubbles A would be diminished, but as described above, there is an upper limit to the diameter of these communicating apertures 13, which limits the amount by which the diameters can be increased.